This type of PLL circuit (hereafter, referred to simply as the “PLL”) is generally structured by a voltage-controlled oscillator (VCO), a phase comparator, and a loop filter. More specifically, the phase comparator detects phase difference between an input signal supplied from the outside thereof and an output signal output from the VCO and feeds back a voltage indicating the phase difference to the VCO through the loop filter. The PLL of this type can be controlled so that the VCO oscillation frequency matches the frequency and phase of the input signal.
A PLL is typically formed as a semiconductor integrated circuit and used in a variety of equipment. For example, PLLs applicable to portable terminals such as portable telephones are described in Japanese Laid-Open Patent Publication No. 2003-152535 (Patent Document 1) and Japanese Laid-Open Patent Publication No. 2003-133951 (Patent Document 2).
Patent Document 1 discloses a PLL employing a VCO operable in a plurality of frequency bands such as GSM (Global System for Mobile Communication), DCS (Digital Cellular System), and PCS (Personal Communication System). The disclosed PLL is effective in that the sensitivity of the VCO control voltage is not increased even if an oscillation frequency band is widened in the VCO and that the PLL is strong against external noise or variation in supply voltage.
Specifically, according to Patent Document 1, an oscillation frequency of the oscillating circuit in each frequency band is measured in a state in which the control voltage of the oscillating circuit forming the PLL is fixed at a predetermined voltage and the measured value is stored in a memory circuit. This measured and stored value of the frequency is compared with a set value which is given during operation of the PLL so as to designate a frequency band. On the basis of the comparison result, a frequency band actually used in the oscillating circuit is determined.
Accordingly, the PLL described in Patent Document 1 comprises, in addition to the memory circuit, a variable frequency divider connected to the VCO, a frequency counter for counting frequency based on both an output from a reference oscillator and an output from the VCO, and a phase comparator for comparing the phase between the VCO output and the reference oscillator output.
Patent Document 2 discloses a PLL which performs in response to a data signal, a clock signal, and a strobe (STB) sent from a CPU. According to the disclosure of Patent Document 2, a signal with a desired frequency can be generated by setting a desired count value in a programmable counter by the use of the data signal.
Patent Document 2 also proposes a PLL provided with a circuit which ignores noise so that malfunction is prevented even if the noise is superimposed on a strobe signal. Specifically, a strobe signal having a predetermined pulse width is generated by the CPU, and reference signals are counted during the pulse width interval of the strobe signal, whereby the strobe signal is discriminated from the noise to prevent malfunction due to noise.
At any rate, Patent Documents 1 and 2 disclose PLLs capable of varying frequency by using a frequency counter or a programmable counter.
However, neither Patent Document 1 nor 2 points out any problems unique to mobile wireless communication devices such as portable terminals have problems and appropriate countermeasures needed in PLLs used in such mobile wireless communication devices so as to solve the above-mentioned problems.
More specifically, it is a common practice in a mobile wireless communication device such as a portable terminal to set a power saving mode while the device is not in conversation mode or it is in standby mode so that the power consumption becomes less in comparison with when the device is in conversation mode. The term “power saving mode” as used herein is a mode which is temporarily executed in operation of mobile wireless communication devices such as portable terminals and is different from the mode which is used for setting.
Further, a portable telephone among portable terminals is individually provided with an integrated circuit portion including a CPU and a clock generator for generating a reference clock and another integrated circuit portion for receiving the reference clock as an input signal to drive a display device such as a liquid-crystal display (LCD). In this case, the integrated circuit portion for driving the display device may be provided with a PLL so that the PLL is operated according to the reference clock.
In the portable telephone having such configuration, it often happens that the reference clock frequency is remarkably reduced in the power saving mode in comparison with that in ordinary conversation mode, or the power supply to the circuit portion including the CPU is sometimes turned off to reduce the reference clock frequency to zero.
Consideration is also made about parallel/serial (P/S) converting an output signal of a PLL operated in response to a reference clock and supplying the converted signal to a display device. This structure makes it possible to reduce the number of wiring lines and to reduce the occurrence of noise among parallel wiring lines. When such a P/S converter is provided, the frequency of a PLL clock output by the PLL becomes much higher than the frequency of a reference clock. Moreover, when the reference clock frequency is reduced or stopped in the power saving mode, variation in the PLL clock frequency of the PLL will become severe.
Accordingly, in portable telephones having a P/S converter, the PLL clock will vary largely when the reference clock frequency is reduced or stopped in the power saving mode. Therefore, it has been found that conventional PLL control circuits provided with a PLL are not able to follow such variation in the PLL clock, resulting in occurrence of malfunction.
However, neither Patent Document 1 or 2 mentions about possible malfunction of the PLL in the power saving mode or about countermeasures to prevent the same.